The molecular beam epitaxy ("MBE") process has been accepted as a preferred fabrication process for the production of Indium Phosphide ("InP") type heterojunction bipolar transistors ("HBT") and high electron mobility transistors ("HEMT"). In that process selected group III and group V elements of the periodic table of elements are heated to vaporization temperatures in respective shuttered fusion cells inside of a vacuum chamber for deposit on a substrate of InP placed within that chamber. The elements that are vaporized at different times typically include Indium, Gallium, arsenic, silicon and phosphorous.
As each element is needed in the fabrication process, a shutter covering the respective heated element, is opened, allowing the vapor of the element, metal or non-metal, to form a beam of molecules of the element that exits the associated fusion cell and spreads into the vacuum chamber. Some of that evaporated material deposits on the InP substrate to build-up epitaxial layers of the material on the substrate, which is the purpose of the process. The materials are evaporated at high temperature and pressure from individual fusion cells inside the chamber, and the hot molecules of the vapor settle on relatively cold surfaces. One by one those reservoirs are uncovered in a defined sequence to create the molecular vapors, and the molecule vapors of each material deposits in sequence on the semiconductor wafer and, unfortunately, on the walls of the chamber. By appropriate control of the fusion cell shutters, the desired layered structure that defines the transistor is ultimately produced. The remainder of the material that was evaporated during the processing, however, settles on the walls of the vacuum chamber, and constitutes the waste or, as otherwise termed, the residue of the MBE process.
Typically, the MBE deposition process continues for long periods. Typically, between four and six months of continuous operation is taken to complete a respective HBT or HEMT transistor structure. Eventually with continued processing, the chemical elements within the fusion cells in the MBE vacuum chamber are depleted, and must be replenished. At that time the heat is turned off, the vacuum chamber is vented to the atmosphere, the chamber door is opened, and the InP substrate removed. The inner walls of the chamber are covered with the residue of the processing. The residue on the chamber wall may comprise chunks or particulate of any and all of the materials vaporized during processing. Two principal ingredients of that residue are particulate arsenic and red phosphorous material.
The access ports to the chamber are opened and the internal chamber walls are scraped and/or abraded of the accumulated solid red phosphorous and arsenic, cleaning the walls. Scraping the chamber walls of those materials creates a potential for exposing maintenance personnel and the laboratory or factory environment to hydride gases and caustic acids in vapor form, recognized as hazardous. This potential gas and acid vapor generation develops predominantly in the event of an ignition and sustained burning of the red phosphorous.
Red phosphor, a metal of an allotropic form referred to as P-4, has quite different chemical properties than arsenic, and different even from the white form of phosphor (e.g. P-2 or white phosphor). Red phosphor is easily ignited by friction, and can burst into flame. Even when the flame is extinguished, the phosphor may continue to smolder, posing a continuing fire danger. Further, one of the products of combustion of the red phosphor when ignited in air is phosphoric pentoxide, a very corrosive gas. Should a fire be detected during cleaning, the chamber must be quickly closed and/or fire retardant applied to the smoldering materials until the fire is extinguished.
To avoid danger to the person, those persons charged with cleaning MBE chambers at present are carefully educated and trained to know what to look for and expect. Those individuals must remain alert and exercise care when cleaning the chamber. Those duties impose an undesirable burden on the individual. In some instances one might foresee that one might fail to visually detect smoldering phosphor, except by inhaling some of the noxious gas produced by the phosphor combustion.
To withstand the high temperature and pressure of the MBE process, the chamber is constructed of stainless steel with thick walls, is very heavy, and is intended to remain in a stationary position within the production facility. To produce InP devices in mass, a production facility typically contains a number of such MBE chambers so that a number of wafers can be processed concurrently.
A containment system constitutes a safety apparatus intended to protect personnel from hazardous gases or material vapors as might be generated in industrial processes by capturing those harmful gases and material vapors at the location of the work and exhausting them to other locations, away from the working personnel, where those noxious gases and vapors may be dissipated harmlessly, detoxified, and/or otherwise and safely handled. To accomplish that function, containment systems typically include a hood and an exhaust conduit, supported in the hood, for connection into an exhaust system or pump that draws gases away from a work area beneath the hood. Often the hood is supported by side and back walls to a work bench at which the personnel work on various workpieces, defining a the three dimensional region or chamber, herein referred to as the confinement region, with a front opening through which personnel may access the work pieces.
In some instances, as in the case of the present invention, a work piece may contain combustible ingredients as might catch fire. The smoke created by the fire is exhausted by the exhaust conduit, but, for the obvious safety of personnel and property, the fire must also be extinguished. Thus containment systems for such kinds of work activity include fire extinguishers.
Those fire extinguishers may be a self-contained portable unit that is universally recognized by the red color. In more sophisticated fire extinguisher systems for this application, liquid fire retardant is pumped through a series of nozzles that are supported in the confinement region and are arranged to direct the fire retardant over the work area to douse the flames. In still more sophisticated fire control systems, ultra-violet and infra-red sensors are used to detect a fire in the confinement region and automatically start the dispensing of fire retardant into the area. The present invention also incorporates such known beneficial features.
Personnel who work with hazardous chemicals at the workbench in research laboratories and industrial facilities in which noxious gases might possibly be generated through unintended combustion of a material have long enjoyed the protection provided by the modern workbench in that industry. Such workbenches are known to contain fume evacuation system, infra-red or ultraviolet fire detection devices and fire extinguishing systems. As an advantage to the present invention brings the foregoing kind of hazard protection to personnel engaged in cleaning of MBE chambers.
Accordingly, an object of the present invention is enhance in the cleaning of MBE chambers.
Another object of the invention is to prevent personnel from being exposed to noxious gases or vapors during cleaning of an MBE chamber.
A further object of the invention is to provide a portable containment system for use when cleaning of an MBE chamber.
A still additional object is to provide apparatus that automatically detects a fire in an MBE chamber and automatically prevents any gases of combustion from escaping into the laboratory or factory.
A still further object of the invention is to confine and prevent any products of combustion as may occur through spontaneous combustion of the phosphor debris removed from the chamber of an MBE processing system from reaching the cleaning technician and to extinguish any such fires automatically.
And an ancillary object of the invention is to provide a containment system design that permits the containment system to be disassembled, moved to another MBE chamber, and reassembled, quickly and easily.